When an audio signal is reproduced for listening such as through speakers or headphones, the resulting audio signal and the listener's perception of the audio signal depend on a number of factors, including such things as the listener's hearing profile, the frequency response of the device used for transduction and production of the audio signals, and other factors (e.g., the listening environment, including noise, room acoustics, different levels of attenuation at different frequencies, etc.). Audio signals are often equalized in order to compensate for some of these factors. Equalization alters the frequency response of an audio signal reproduced using an electronic means of transduction by filtering of the audio signal. For example, if the listener has hearing loss in certain audio bands, signals in these bands can be amplified to recreate natural listening conditions intended for the audio signal.
Each listener's hearing profile differs depending on various factors, such as age, psychoacoustic factors, existence of hearing loss, and if present, type of hearing loss. Furthermore, a variety of devices exist for listening, including in-the-ear earphones that seal the ear from external noise, earbuds that are placed in the ear but do not provide a seal, circumaural or over-the-ear devices, supra-aural or on-the ear devices, open-back devices, and stereo speakers, to name but a few, plus the related devices that process and amplify the audio signal, including MP3 players, smartphones, pad computers, laptop computers, desktop computers, stereo receivers, and other similar devices. The frequency response of the signal path between the receiver of the listening device and the listener's tympanic membrane also varies and is dependent on the frequency response of the listening device and the volume of air in the signal path. The frequency response of this signal path therefore varies from listener to listener. The electronic amplifier used to deliver power for transduction of the audio signal also has a characteristic frequency response. Thus, each of the devices within a system that reproduces an audio signal for listening has a unique characteristic frequency response that shapes the audio signal being reproduced as does the listener's hearing profile and listener-listening device combination.
Prior art provides equalization based on a standard audiogram, which is a threshold hearing level test that is performed using specialized audio equipment in an audiometric booth. Specifically, the threshold of hearing is established through presentation of pure tones and listener indication of whether the tone is heard at each frequency. The threshold at which each tone is heard at each frequency establishes the audiogram. The zero phon (threshold) level equal-loudness contour may be used to provide an equalization filter to enhance hearing. However, in the context of personalized audio listening, measuring the audiogram requires an environment with a very low noise floor, such as an audiometric booth, and external noise influences the accuracy of threshold determination significantly. Moreover, audiograms are generally performed with a specified listening device, and not the listening device preferred by the user for personal use in listening to audio. Furthermore, it is well-known that the frequency response of human hearing is itself a function of sound pressure level, and thus that the sensitivity of hearing is strongly frequency- and level-dependent. As such, audiometric testing provides only a single measure of frequency response of human hearing—at threshold. Since listening generally does not occur at threshold, the equalization filter generated through threshold tests provides an inadequate representation of equalization needed at higher listening volumes to restore an equalized signal providing natural listening matched to a specified equal-loudness contour.